1. Field
The present invention relates to an image forming apparatus to perform target value correction processing to stabilize a development density by correcting a control target value of a control parameter, which influences a developing ability of an imaging unit.
2. Discussion of the Related Art
The image forming apparatus described in JP-H09-006120-A has been known as this type of image forming apparatus. In the case where a predetermined retention time or more has passed, the image forming apparatus corrects a control target value of toner concentration of a two-component developer used for development as a control parameter, which influences a developing ability of an imaging device including a photoreceptor and a developing device. Specifically, the retention time is a time period during which an image forming operation is continuously stopped. During the retention time, the two-component developer in the developing device is left to stand without being stirred. As the retention time becomes longer, toner charge amount (Q/M) of the two-component developer decreases. As a result, the output from a toner concentration sensor shifts from the actual toner concentration to a lower concentration, which leads to misdetection of low toner concentration. Then, in the case of the image forming operation performed after the predetermined retention time or more, the image forming apparatus described in JP-H09-006120-A performs target value correction processing for correcting the control target value of the toner concentration to a value lower than a normal value. As a result, the toner concentration can be kept at an appropriate value even after the predetermined retention time or more.
As a result of intensive studies, the inventors have found that, after a predetermined retention time or more, it is desirable to correct a control target value of toner concentration not only because the output from the toner concentration sensor shifts to a lower concentration than usual but also for the reasons below. That is, when the retention time becomes quite long, the toner charge amount (Q/M) of the two-component developer decreases to a significant level. Then, electrical adhesion of toner particles attracting to the carrier in the two-component developer is considerably weakened, which causes the toner particles to easily detach from the carrier surface and adhere to the electrostatic latent image. As a result, the developing ability of the imaging device including the photoreceptor and the developing device becomes high so as to make the development density excessively dense. Thereafter, when the image forming operation is performed thereafter, the two-component developer is stirred in the developing device, which promotes triboelectric charging of toner. Thus, the toner charge amount (Q/M) starts to increase, but the development density is still rather dense until the toner charge amount (Q/M) reaches a high value to some extent. Therefore, in the image forming operation after the predetermined retention time or more, it is desirable to make the development density appropriate by decreasing the control target value of the toner concentration lower than the actual value to decrease the developing ability.
However, it has been found that, once the development density is made appropriate in the above manner, the image forming apparatus is likely to have a poor development density in future. Specifically, when the image forming operation is performed after the target value correction processing is performed in the state where the toner charge amount (Q/M) has decreased after a long retention time to greatly decrease the control target value of the toner concentration, the toner charge amount (Q/M) of the two-component developer starts increasing to an average value. Since the developing ability will be too low in due time, it is required to raise the developing ability by increasing the control target value of the toner concentration. However, even if the control target value of the toner concentration is increased, it takes a certain period of time for the toner concentration of the two-component developer to increase to the corrected control target value. Since the toner concentration of the two-component developer is much lower than the appropriate value during that period of time, the developing ability is too low to achieve a sufficient development density.
Meanwhile, not only in the case where the image forming operation is performed after a long retention time, but when the control target value of the toner concentration is greatly corrected, it takes a certain period of time for the toner concentration of the two-component developer to reach the same value as the corrected control target value. During that period of time, the development density may be excessive or may be too low. For example, temperature and humidity influence the developing ability of the imaging device. This is because, as temperature or humidity becomes lower, the triboelectric charging property of toner in the two-component developer becomes higher to increase the toner charge amount (Q/M). Therefore, when the temperature and humidity rapidly changes due to, for example, starting of an air conditioner, it is desirable to correct the control target value of the toner concentration according to the temperature and humidity change. However, even though the control target value of the toner concentration is greatly corrected in accordance with the great change in the temperature and humidity, the toner concentration still greatly deviates from the appropriate value until the toner concentration of the two-component developer reaches the corrected control target value. As a result, the development density becomes excessive or too low.
Moreover, the occurrence of a time lag between a great correction of the control target value and the reflection of the correction on practical developing ability of the imaging device is not limited to the correction of the control target value of the toner concentration. For example, it is assumed that the control target value of the laser intensity of a laser optical system that optically writes an electrostatic latent image to a clear background surface (hereinafter just “surface”) of the photoreceptor is greatly corrected. In this case, the electric potential of the electrostatic latent image to be written after the control target value of the laser intensity is greatly corrected can be corrected to the target electric potential. However, an electrostatic latent image has already been written on the surface of the photoreceptor before the control target value of the laser intensity is greatly corrected. Specifically, the electrostatic latent image is optically written on an optical writing position at a predetermined position in the circumferential direction of the photoreceptor, and then, as the photoreceptor rotates, the electrostatic latent image enters a development area opposite from the developing device and is developed. In the case where the control target value of the laser intensity is greatly corrected, the electrostatic latent image which has already entered the developing area and the electrostatic latent image which is present between the optical writing position and the development area at the time of correction are written with the uncorrected laser intensity, and therefore, their electric potentials deviate from the target value. As a result, a certain time lag exists from when the control target value of the laser intensity is corrected and until the electrostatic latent image written with the corrected laser intensity is developed, i.e., until the correction is reflected on the practical developing ability of the imaging device. As such, some control parameters of the image forming apparatus cause time lags between when the control target values are corrected and when the corrections are reflected on the actual developing ability of the imaging device.